1. Field of the Invention
The present invention relates to a gas sensor usable for combustion control of an internal combustion engine for an automotive vehicle.
2. Description of the Background Art
A gas sensor is generally installed in an exhaust system of an automotive engine to use a sensing signal for combustion control. For example, as disclosed in U.S. Pat. No. 5,288,389, this kind of gas sensor includes a multilayered gas sensing element made of ceramic members for detecting the concentration of a specific gas contained in the exhaust gas.
In general, the multilayered gas sensing element includes a predetermined number of thin ceramic substrates stacked or laminated on one another. The ceramic substrates are weak in mechanical strength and therefore may be broken when subjected to vibrations or shocks.
Especially, the weakest portion of the gas sensor is a protruding end portion of the multilayered gas sensing element protruding from a front or distal end of the insulator.
Shortening the protruding length of the multilayered gas sensing element is effective to eliminate such damage. However, the protruding end portion of the multilayered gas sensing element is a portion where a sensing portion is provided to detect the gas concentration. The gas concentration sensing portion needs to be exposed to a measuring objective gas in a measuring objective gas side cover. To assure accurate detection of the gas concentration, it is necessary to maintain the temperature of the gas concentration sensing portion. This is why the heater is integrally provided with the gas sensing element, or separately provided, to heat the gas sensing element.
If the protruding length of the multilayered gas sensing element is shortened, the heat to be used for warming up the gas sensing element will leak via the insulator and the housing. In other words, it will be difficult to maintain the temperature of the gas concentration sensing portion to a constant value, as it is located in the protruding portion of the gas sensing element. To avoid this, a minimum protruding length must be secured. Hence, the protruding length of the gas sensing element cannot be shortened so much.
Nowadays, the multilayered gas sensing elements used in the automotive engines or the like are required to have excellent warm-up or activation properties. In general, this kind of multilayered gas sensing elements cannot operate normally to detect the gas concentration unless the temperature reaches the activation level. When the combustion control mechanism must start its operation immediately after the automotive engine starts up, it is definitely necessary to accurately detect the gas concentration as quickly as possible. To this end, the multilayered gas sensing element is heated by a heater to promptly increase the temperature of the element to the activation level.
To realize such immediate or prompt activation of the gas sensing element, reducing the size of the multilayered gas sensing element is effective in that the heat capacity decreases.
However, incorporating a compact multilayered gas sensing element into a gas sensor will encounter with the following restrictions.
Referring not to an example of the present invention (FIG. 1) which will be later explained, this type of gas sensor 1 includes a multilayered gas sensing element 2 fixed to a housing 10.
The multilayered gas sensing element 2 includes electric terminals (265 and 266 as shown in FIG. 2) for supplying electric power to this element 2 and for outputting a sensing signal from this element 2. Plate terminals 131 are electrically brought into contact with these terminals (refer to FIG. 1).
To assure insulation between plate terminals 131, the multilayered gas sensing element 2 needs to be inserted into an insulator 3 and securely fixed to this insulator 3. Then, the assembly of the multilayered gas sensing element 2 and the insulator 3 needs to be inserted into the housing 10 and securely fixed to this housing 10.
In this case, a seal member 30 is provided to fix the multilayered gas sensing element 2 and the insulator 3. To secure the strength of the seal member 30, the width of the multilayered gas sensing element 2 cannot be reduced so much. In general, the required number of the plate terminals 131 is four, i.e., two at the front side and two at the rear side according to the illustration of FIG. 2. To assure the insulation between these plate terminals 131, the width of the multilayered gas sensing element 2 cannot be reduced so much. On the other hand, there are no specific restrictions regarding the downsizing of the gas sensing portion.